There are a large variety of inks which are available for use on surfaces, such as metal, fabrics, wood, glass or plastics. Inks in general consist of a vehicle, or carrying agent, and a colorant that is evenly dispersed throughout the vehicle. One particular example of a type of ink is flexographic inks (formerly named aniline inks) which are used on presses with rubber printing plates. Flexographic inks are being used increasingly, especially for package wrappings such as foils, transparent plastic films, or paper-bag machines. They are generally composed of volatile solvents such as low boiling point alcohols, esters, aliphatic and aromatic hydrocarbons, ketones and water.
The most widely used family of flexographic inks are formulated from polyamide resins. Polyamides are formed by combining carboxylic acids, mostly dibasic, with organic polyamines, usually diamines. The acid and amine groups immediately react to form a salt. Upon heating to 140.degree. C. or higher, this salt decomposes with the evolution of water to give an amide bond.
Alcohol soluble polyamides are widely used in alcohol based flexographic inks for printing on plastic film. Environmental concern over the amounts of volatile organic solvents in the atmosphere has led to a desire to use aqueous solutions that have less volatile organic solvents contained therein. In order to meet new Environmental Protection Agency regulations, it is desirable to employ water based flexographic inks with reduced levels of volatile organic solvents. In order to accommodate the reduced levels of volatile organic solvents, the polyamide resins used should have increased water solubility and yet retain other desirable properties of polyamide resins. The major technological difficulty has been in making water dispersible (WD) polyamides which provide inks with good properties such as adhesion, gloss, water resistance, and blocking resistance.
To achieve water dispersibility, it is necessary that the polyamide have a high acid value (AV) in the range of 50-100. When the free acid groups of the resin are neutralized with ammonia, it becomes water soluble. After printing, the ammonia evaporates and the resin develops water resistance.
Making a high AV polyamide using standard synthetic methods presents no problem. It is simply a matter of using a large excess of carboxylic acid over amine in the formulation. The difficulty is that polymer molecular weight is inversely proportional to AV. Therefore, if standard synthetic methods are used, WD polyamides with AV=50-100 are much lower in molecular weight than conventional alcohol soluble polyamides, which usually have acid values of less than 10. This lower molecular weight results in soft, sticky resins with degraded performance. Therefore, the problem is synthesizing polyamides with both high AV and good hardness.
Polyamides which are rendered water dispersible have been described in the prior art literature:
U.S. Pat. No. 3,776,865 to Glaser and Lovald discloses polyamide resins obtained by reacting an acid component comprised of a polymeric fat acid and another dicarboxylic acid with an amine component comprising isophorone diamine or mixtures thereof with an alkylene diamine. At least 12.5 carboxyl equivalent percent of the polymeric fat acid is employed. The patentees disclose that these resins are useful as binders applied by aqueous systems, particularly in flexographic/gravure inks where water reducibility is desired.
U.S. Pat. No. 3,778,394 to Lovald and Glaser discloses that the acid used to make the water dispersible polyamide is largely composed of a rosin acid-carboxylic acid adduct.
U.S. Pat. No. 4,514,540 to Peck, discloses that included in the starting materials of the water dispersible polyamide is a preformed synthetic resin having carboxyl and/or hydroxyl groups.
U.S. Pat. No. 4,683,262 to Whyzmuzis and Menke discloses a method where little or no polymeric fatty acids are used to make the polyamide.
In spite of the wide variety of polyamide containing water dispersible compositions known through the prior art descriptions, there remains a need for improved polyamide compositions which are water dispersible and yet retain adhesion, gloss, water resistance and blocking resistance properties, characteristic of traditional alcohol soluble polyamides.
Polyamides can also be utilized in Cold Seal Release Lacquers which are used in packaging. Cold Seal Release Lacquers (CSRL) were originally developed for the candy bar/chocolate market. Cold seal technology has now expanded into the high growth flexible snackfood packaging industry. Cold Seal Release Lacquers are generally a polyamide or nitrocellulose/polyamide blend system designed to act as a protective coating for the printed side of a film package. The CSRL must provide gloss and scuff protection to the finished product as well as being block resistant while the printed film is in roll form. High wind-up tension within the printed roll makes the CSRL/Cohesive interface critical. If the CSRL does not provide a smooth, easy unwind, ink picking, film tearing or difficult machining will occur.